Receiver noise limiter circuit



May 3, 1938. w. VAN B. ROBERTS I RECEIVER NOISE LIMITER CIRCUIT Filed Oct. 30, 1936 A A A A A A iT, h

INVENToR WALTER VAN ROBERTS BY Z f MW ATTORNEY Patented May 3, 1938 UNITED STATES PATENT OFFICE Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 30, 1936, Serial No. 108,427

6 Claims.

My present invention relates to improvements in noise limiting arrangements for radio receivers, and more particularly to signal-controlled noise limiters for receivers.

I have disclosed, and claimed, in my application Serial No. 80,497, filed May 19, 1936,- a quickacting vnoise limiting device for a radio receiver, Such a noise control device is normally biased in such a manner that it has no effect on the detector as long as the peak voltage at the detector input circuit is less than the bias on the limiter'device. When undesired voltage impulses of very short duration, but of amplitude in excess of said bias, are impressed on the detector input f* circuit, the limiting device is rendered operative to disable the detector. Such voltage impulses might be produced by spark plugs; static; lightning; and in general, are those which shockexcite the sharply tuned resonant circuits of the receiver to produce relatively slowly dying-out trains of oscillations which upon rectification produce large pulses of current in the loudspeaker.

In my aforesaid circuit the bias on the limiter device, and hence the signal peak voltage at which the detector is disabled, is determined by a manually adjustable bias source. For different carrier strengths it is necessary to change the bias adjustment. The receiver operator must, of necessity, concern himself with the proper adjustment of the limiting device in such a construction.

Accordingly, it may be stated that it is one of the main objects of my present invention automatically to regulate the biasing of the noisel limiter device in accordance with carrier amplitude variation thereby eliminating the need for manual adjustment of the limiter bias.

Another important object of the invention is to suppress the effect of short duration-high amplitude transients on a radio receiver; this sup pression being accomplished by automatically rendering the detector ineffective by a suppression device when the detector input circuitpeak voltage exceeds a normal bias on the device, and the received signals being employed to maintain the supression device ineffective until the bias thereof is exceeded.

Another object of the invention is 4to provide a receiver of the type including a noise limiter device arranged to prevent signal detection when the detector input peak voltage overrides a signalderived bias on the limiter device; there being means provided, in addition, to enable the operator to connect, or disconnect, the limiter device at will whereby the noise limiter device is (Cl. Z50-20) used solely when noise impulses are of suflicient amplitude to mar reception.

Still other objects of the invention are to im'- prove generally the operating ease and efficiency of noise limiters for receivers, and more especially to provide receivers which are not only reliable and simple in operation, but include noise limiter devices which are economically manufactured and assembled in radio receivers.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in Iconnection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing: 1

Fig. lshows a receiving circuit, in, diagrammatic form, embodying one form of the invention;

Fig. 2 shows a modification of the invention.

Referring now to the accompanying drawing, wherein like reference characters in the different figures designate similar circuit elements, Fig. l shows aconventional type of superheterodyne receiving system. It will be understood that this receiver comprises the usual and well known networks that are ordinarily encountered in such types of receivers. For example, the receiving system comprises a signal source, l, such asia grounded antenna circuit, which feeds collected signals to the tunable radio frequency amplifier 2. The amplifier 2 will employ one, or more, amplifier tubes, and utilizes the usual variable tuning condenser. The amplified signals are fed to the tunable input circuit of the first detector section of the converter network 3. The converter may be of the combined local oscillatorfirst detector type employing the pentagrid co-nverter tube of the 6A7 type; or, separate first detector and local oscillator tubes may be used. In any case, it will be clear that the tunable circuits of thevradio frequency amplifier, the first detector and the local oscillator will have the rotors of the variable tuning condensers arranged for mechanical uni-control adjustment of tuning.

The I. F. output of the converter 3 is impressed upon one, or more, stages of I. F. amplification, and the numeral 4 is to be understood as comprising one, or more, I. F. amplifier tubes; each tube having associated therewith a resonant network whch is iiXedly tuned to the operating I. F.

' The last I. F. output circuit 5 is coupled to the I. F. input circuit 6 of the second detector, or audio demodulator, device 1. The latter is of the diode type, and has its anode 9 connected to a predetermined point on the input coil 8 through a path which includes a condenser and the adjustable tap I2. The cathode I3 of diode 1 is connected to the low alternating potential side of input circuit 6 through a path which includes the resistor I4, it being noted that the low alternating potential side of circuit 6 is grounded.

The detector load resistor I6 is connected in series between Ythe anode 9 and cathode I3, and is in series with the resistor I4. 'Ihe condenser I1 is connected in shunt withz resistor I4. The audio component of rectified signal energy flow through resistor I6 is transmitted to one, or more stages of audio amplification through a path which includes condenser 20 and the tap 2 I, and the latter may be adjusable for manual volume control purposes. The audio network will generally terminate in a reproducer of .any desired type. The direct current component of the rectified signal voltage appearing across resistor I6 is transmitted, as an AVC bias, to the stages preceding the second detector, and which prior stages have their gains under control.

As shown in Fig. 1, the AVC network comprises the lead 30 which is connected to the anode side of resistor I6; the AVC bias is. transmitted through filter resistors, denoted by the numeral 4E, in order substantially to suppress the pulsating components of the rectified I. F. energy. Those skilled in the art will readily understand that the connections from the lead 30 are made to the various signal grid circuits of the radio frequency amplifiers, the first detector and the I. F. amplifiers; condenser 6I) being chosen to give the AVC filter a suitable time constant. The AVC network functions to decrease the gain of each control tube as the received signal amplitude increases. In this way the signal amplitude at the input circuit S is maintained substantially uniform regardless of variations at the collector I. In the absence of incoming signals the serrsitivity of each of the stages 2, 3, and Y4 will be a maximum, but under normal operating conditions the gain of each regulated'vstage is reduced, by the AVC action, from the maximum value.

A second diode 5t .is e1eetriea11y associated with the second detectorcircuit. The anode of diode 50, which diode functions as the noise squelcher or limiter, is connected to a point 5I on `the input coil 8 through a condenser52. The point 5I is at a higher alternating current potential than the point 53 to which the anode 9 is connected. The cathode of diode 50 is connected to the detector cathode side of resistor I4. A biasing device is provided for the squelch diode 50, and this biasing device comprises the diode 10 whose anode 1I is connected to the anode of diode 50 through a radio frequency choke coil 12. The high alternating potential side of input circuit 6 is connected to the anode 1| through a condenser 13, and the yanode and cathode of diode 10 are directly connected by a resistor 14. The cathode of diode 10 is at ground potential.

To explain the operation of the noise limiting device used in this arrangement, let it be assumed that the lead I2 has been adjusted to such a point on coil 8 that when 100 volts carrier peak are impressed across the entire circuit 6 then only 30 volts carrier peak exist between the anode 9 and cathodeA I3. In other words the anode 9 is tapped down on coil 8 to a point such that there is a ratio of approximately 10:3 between the entire input voltage and the voltage impressed across the detector diode. The sqeulch diode is now adjusted by connecting its anode to the point 5I on coil B which is at a lesser alternating current potential (say 70 volts) than the peak voltage across the entire input circuit.

The full carrier voltage across input circuit 6 is applied to diode 10, and with large values of condenser 13 and resistor 14, the negative direct current potential on the anode of squelch diode 59 is nearly equal to the peak amplitude of the signals. This results, in other words, in a biasing of the anode of diode 5U negatively with respect to its cathode by the voltage drop occurring across resistor 14 due to space current flow through diode 1I. Since the anode of diode 50 is tapped down to point 5I on the input coil 8, the peak I. F. amplitude applied to the anode of diode 50 through condenser 52 is less than the bias on diode 50, and, therefore, no current can be drawn by the squelch diode 50.

If a high amplitude transient, of short duration occurs, it causes current to flow through diode 50, and hence through resistor I4, Vand accordingly will block detector diode 1. This blocking action on the' detector diode 1 ceases before thel bias on diode 50 generated by diode 10 can increase appreciably. It will, therefore, be seen that the bias generated by diode 10 is automatically just a little more than required to cut off current through squelch diode 50 in the presence of normal signals. In my aforesaid application this latter condition was attained only by a manual adjustment. In the present invention the bias produced by diode 10 prevents current from flowing through the squelch diode 50 in the absence of the aforesaid transients.-

To review the effect of diode 50 on detector diode1, it is pointed out that when the alternating current voltage amplitude across input circuit 6, and transmitted through condenser 52, exceeds the negative bias on the anode of diode 50, current flows through the diode and renders the cathode side of resistor I4 positive with respect to ground. This means that cathode I3 of diode 1, which is connected to the same point on resistor I4, becomes positive with respect to ground, and even positive with respect to anode 9 if the impulse is sufliciently strong, thereby preventing current from fiowing through detectorl 1. If one considers the relation between detector output voltage and input voltage, it will be seen to increase substantially linearly with signal input up to peak volts across the tuned circuit 6, after which it will fall rapidly to zero for further increase in input voltage provided the bias on diode 50 remains constant at 100 volts.

The condenser I1 is of `a magnitude such as to provide with resistor I4 a noise suppressing action which is relatively quick, but short-lived. In other words, the diode 50 is part of a quickacting AVC arrangement which functions to bias the second detector beyond cut-off when the alternating current voltage across input circuit 6 rises momentarily considerably above a predetermined amplitude. 'I'he magnitude of condenser I1 is chosen so that it will retain its charge only sufficiently long to permit the transients of the noise pulse to die down. A suitable time constant of 0.001 second would result from making resistor I4, 0.1 megohm and condenser I1 equal to 0.0005 microfarad. The eciency of detector operation is not impaired by tapping down on coil 8. On the contrary, since the squelch diode 50 normally draws no current, this tapping down coil serves to provide a good impedance match between diode i and circuit 6, and thus no loss `of detector output need result except for the Voltage developed across resistor I4.

The speed of cut-off of detector 1 is controlled by the position of point 53 on coil 8, as well as by the bias imposed on diode Sil by the diode Til. The position of point 5| on coil 8 kwill determine the bias overriding point. The lower the tap I2 on coil 8 the greater will be the speed of cut-Oli. Furthermore, the less bias imposed on diode 50, the faster will be the cut-off action. It is, additionally, pointed out that while diode 1 and 50 have been shown as being separate devices, the electrodes thereof can be disposed within a single tube envelope, and a tube of the GHG type may be used for this purpose. Again, since the cathodes of diodes 'l and 50'are connected to a common point of resistor lll, a tube using a common cathode and two anodes may be Aemployed in place of these diodes, and a tube of the 85 type may be used for the purpose. However, in such a case electrostatic shielding between diode elements is desirable. Of course, it is also possible to dispose the electrodes of the three diodes 1, 5B, and 10 within a single tube envelope. L

In Fig. 2 there is shown a modication of the invention wherein similar, but not quite so gdesirable, results may be obtained. In-this modiiication the diode 10 is dispensed with, and the diode 58 is utilized both for the s quelching function, as well as for the development of the bias which is used to impair the squelch action. In other words, in this modification the diode 50 employs received signals to bias itself to prevent squelching the detector diode 1 on normal signals. The large condenser 80 and high resistance 8i generate a bias on diode 50 that keeps current from flowing through the diode except on modulation peaks following lesser peaks. Strong sudden transients override this bias and cut off the current iiow through detector 1 as explained in connection with Fig. 1. However, the normal signals are slightly affected by reducing the modulation peaks, or sudden modulation peaks occurring after a long interval of low modulation.

In this modification it will be observed that the high resistance 8l is connected between the anode of diode 5G and the grounded side of resistor i4. The large condenser Bil is connected between the anode of diode 50 and a terminal of a double pole-double throw switch device, A switch device is employed to enable the receiver operator to dispense with noise suppression action if he feels that it is not needed. If the noise is bad enough to require the use of the suppressor diode 50, a slight impairment of the tone will not be serious. Hence, by virtue of the use of the suppression selector switch, the modification shown in Fig. 2 is rendered practicable, since the diode 5D can be thrown out of operation when it is not needed and thus the tone of reproduced signals will not be impaired.

The switch device in Fig. 2 is schematically represented. With the adjustable element 90 actuated to its up position the suppressor diode 5D is not employed, and the detector 1 is tapped to the optimum point. This follows from the fact that in this position of the adjustable switch element 9E! the condenser Il is connected to lead 9|, the latter terminating at a relatively high alternating potential point on input coil 8. The

condenser 80, however, will be. connected to .the dummy contact 92. When the element 90 is connected to the down position, the condenser Il is connected to the lower tap point 53, while condenser 80 is connected to the high alternating potential side of input circuit 6. In this latter position of the switch element 90 the diode 50 will perform its suppression function.

' While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled inthe art that my Vinvention is by no means limited to the par'- ticular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

l. In combination'with a source of signals and a load circuit, a diode detector, a resonant input circuit tuned to an operating input frequency coupled to said signal source, means for impressing a predetermined fraction of the voltage across said input. circuit upon` the diode detector electrodes, means electrically associated'with the detector electrodes for developing arectied current lfrom the impressed input voltage, a second diode having Vits electrodes connected acrossv a' greater portion of said input circuit whereby a greater voltage derived from the input circuitu is impressed between the second diode electrodes, an impedance in the space current paths of both diodes having one side thereof connected in cornmon to the cathodes of both diodes, the other side of the impedance being connected to the detector diode anode, and means, responsive to a voltage derived from said input circuit, for producing an initial potential difference between the electrodes of the second diodesuch that the latter has no current flow in the absence of input circuit voltage above a predetermined value.

2. In combination with a source of signals and a load circuit, a diode detector, a resonant input circuit tuned to an operating input frequency coupled to said signal source, means for impressing a predetermined fraction of the voltage across said input circuit upon the diode electrodes, means electrically associated with the detector electrodes for developing a rectiiied current from the impressed input voltage, a second diode having its electrodes connected across a greater portion of said input circuit whereby a greater voltage derived from the input circuit is impressed between the second diode electrodes, an impedance in the space current paths of both diodes having one side thereof connected in common to the cathodes of both diodes, the otherA side of the impedance being connected to the detector diode anode, means, responsive to a voltage derived from said input circuit, for producing an initial potential difference between the electrodes of the second diode such that the latter has no current ow in the absence of input circuit voltage above a predetermined value,

and 'said last means comprising a third diode.l

diode having its electrodes connected across a greater portion of said input circuit whereby a greater voltage derived from the input circuit is impressed between the second diode electrodes, an impedance in the space current paths of both diodes having one side thereof connected in common to the cathodes of both diodes, the other side of the impedance being connected to the detector diode anode, means, responsive to a voltage derived from said input circuit, for producing an initial potential diiference between the electrodes of the second diode such that the latter has no current flow in the absence of input circuit Voltage above a predetermined value, and means for selectively connecting, or disconnecting, the second diode from said resonant input circuit.

4. In a detecting circuit for radio signals, a detector diode, means for supplying alternating voltage to said diode, a second diode, and means for supplying to said second diode an alternating voltage proportional to, but greater than, the voltage applied to said detector diode, means, responsive to said first supplying means, for applying to said second diode a direct current bias voltage greater than the peak alternating voltage which is supplied to said second diode coincidental with normal signalvoltage being supplied to said detector diode, means for biasing the anode of the detector diode negative relative to the cathode in proportion to the direct component of current flowing through said second diode Whereby the application of alternating current voltage suiciently greater than normal to said detector diode results in paralyzing of said detector diode, and means for maintaining said paralysis only for a time subsequent to the cessation of said abnormally high voltage; 1

i 5. In combination, a diode detector with means for impressing a modulated radio frequency voltage thereupon, an output circuit for developing rectified current therefrom, an electron discharge device, and meansfor impressing a larger modulated radio frequency voltage upon the latter for rectification, means for biasing said detector from the rectified current flowing in said device, and means for automatically biasing said device toV an average value substantially equal to the maximum peak radio frequency applied to said device during a cycle of modulation whereby said device is ineffective to bias said detector during a cycle of modulation butdoes bias said detector during transient voltages of considerably higher voltages than any signal voltages within a cycle of modulation.

6. In combination with a source of signals and a load circuit, a diode detector, a resonant input circuit tuned to an operating frequency coupled to said signal source, means for impressing a predetermined fraction of the Voltage across said input circuit upon said detector electrodes, a second diode having its electrodes connected across a greater portion of said input circuit whereby a greater voltage derived from the input circuit is impressed between the second diode electrodes, an impedance common tothe space current paths of both diodes, and means,- responsive to a Voltage derived from said input circuit, for producing an initial potential difference between the electrodes of the second diode such that the latter is substantially non-conductive in the absence of in put circuit voltage above a predetermined value.

WALTER VAN B. ROBERTS. 

